High-pressure pumps having reciprocating plungers, or pistons, are frequently used in oil and gas production fields for cementing, acidizing, fracturing, and other treatments of wells to enhance or restore the production rate thereof. Such pumps are routinely called upon to pump two-phase slurries down the well bore at high pressures. The term, two-phase slurries, refers to a liquid-solid mixture having a preselected amount of solid particles suspended in a liquid, such as sand and water.
The fluid, or slurry, is compressed in what is referred to as the fluid-end of the reciprocating pump which generally includes a cylinder head having an intake valve and a discharge valve. A piston, or plunger, reciprocates in a cylinder that is attached to the cylinder head thereby compressing the fluid or slurry. The pressurized liquid, or slurry, is then allowed to exit the pump through the discharge valve whereupon it is guided into the well-bore. The operating pressures of the fluid end of such a reciprocating pump often reach pressures exceeding 10,000 lbs/in.sup.2 (700 kg/cm.sup.2). Should a leak occur in the cylinder-head and/or cylinder housing passages in the fluid end of the pump, the highly abrasive nature of two-phase slurries under such pressures, quickly erodes the fluid end body causing a multitude of problems. Namely, system pressure loss, spillage of the slurry about the location of the pumping equipment, and costly down time of the pumping equipment when it is either repaired or replaced. Costs associated with down pumping equipment include the repair costs of the pump, the cost of which depends on whether the pump is repaired in the field or shipped to a repair facility, the idle time of the crews needed to perform the treatment, and the loss of production revenue generated by the oil or gas well being treated.
One means of combatting such cylinder-head leaks is to attach the cylinder-head to the cylinder and/or the crankcase by bolts, or studs, that are tightened to preselected torques to induce a relatively high preload thereon. Such an arrangement is shown in U.S. Pat. No. 3,373,695--Yohpe which relates to reciprocating pumps and is representative of the prior art. FIG. 1 herein, is a reproduction of FIG. 3 of Yohpe '695, which depicts a discharge manifold 21 having wall member 45 and cover 46 being secured to cylinder block 29 by co-acting studs 48 and nuts 47. FIG. 2 herein, is a reproduction of FIG. 4 of Yohpe '695 which shows an alternative fluid end assembly having cover 46 directly secured to block 29 by studs 48 having nuts 47 threaded thereon.
A problem inherent with the prior art arrangements is that relatively large diameter bolts, or studs, are required that can withstand the high cyclic tensile loads induced by the plunger compressing the fluid within the fluid end of the pump. Furthermore, the bolts, or studs, must be accurately tightened to prescribed torques to induce a relatively high preload on the bolts, or studs. Consequently, if a cylinder-head is removed in the field for repairs to be performed on the pump, it is very probable that the bolts, or studs, retaining the cylinder-head will not be re-tightened accurately. That is, the bolts or studs are likely to be under or over tightened by those making the repairs in, more often than not, harsh field conditions. Such improper tightening can result in further seal failure with all of the associated problems previously mentioned. Thus, there is a long standing need to provide a pump fluid end for reciprocating pumps that provides a cylinder-to-head seal that is less sensitive to having cylinder-head bolts or studs being improperly tightened without jeopardizing the integrity of the seal arrangement between the cylinder-head and the cylinder housing.
Furthermore, the usage of overly large diameter bolts can result in design limitations in the cylinder head, or block, and the mating cylinder due to size considerations.
Another enduring problem within the industry is that the solid particle portion of the slurry, under certain operating parameters, can become packed in the fluid end of the pump resulting in an over-pressure situation that can be extremely damaging to certain structures within the pump, including for example, the cylinder-head structure, the piston or plunger, as well as the connecting rod and crankshaft assembly.
A variety of solutions to prevent over-pressure induced damage to pumps have been devised. One such solution is taught in U.S. Pat. No. 5,073,096--King et al. assigned to the assignee of the present invention. The '096 King et al. patent discloses a front-discharge fluid end for a reciprocating pump wherein a fluid outlet valve is positioned co-axially with the plunger assembly of the pump. Thus, the pump disclosed in the '096 patent allows solid particles that have precipitated out of the two-phase slurry within the pump to be nonetheless discharged through the outlet valve preventing over-pressure induced damage to the pump. Notwithstanding the solution disclosed in the '096 King et al. patent, there remains a need in the art for a pump fluid end assembly and associated method that provides yet further protection of the structure and components of a reciprocating pump should an over-pressure condition arise.